A polypropylene sheet has been finding an increased number of uses in various fields because of its low production costs as well as excellent mechanical and optical properties, secondary moldability and moisture proofness. In some applications, however, these properties are not sufficiently satisfactory. For example, in fields where the polypropylene sheet competes with those of, for example, polystyrene and polyvinyl chloride, it is required to be improved in stiffness and transparency.
Several attempts have been made to improve the siffness and transparency of polypropylene. It is known that if aluminum or sodium salts of aromatic carboxylic acids (Japanese Patent Application (OPI) No. 80329/83), or aromatic carboxylic acids, aromatic phosphoric acid metal salts, and sorbitol derivatives (Japanese Patent Publication No. 12460/80 and Japanese Patent Application (OPI) No. 129036/83) are added, they act as agents to form crystal nuclei (hereinafter referred to as "nucleating agents"), thereby improving the stiffness and transparency. The term "OPI" as used herein means a "published unexamined Japanese patent application".
Of the above nucleating agents, the sorbitol derivatives are excellent in the nucleating effect. These sorbitol derivatives, however, have disadvantages in that they readily bleed out of the polypropylene after molding, thereby causing whitening of the molded polypropylene, and they generate unpleasant odor during the molding. Thus the sorbitol derivatives are limited in their application. Further, the aluminum salts of aromatic carboxylic acids are widely used as the nucleating agent. However, polypropylene sheets containing the aluminum salts, when allowed to stand for long periods of time, may absorb moisture and cause turbidity.
Japanese Patent Publication No. 32430/70 discloses that copolymerization of a random copolymer of propylene and an .alpha.-olefin having 4 to 8 carbon atoms with 3-methylbutene-1 improves transparency. In this case, however, the effect of improving the transparency is not sufficiently high.